CAP theorem

The CAP theorem, also known as Brewer's theorem, is a concept in distributed computing that outlines the trade-offs between three crucial properties—Consistency, Availability, and Partition Tolerance. Proposed by Eric Brewer around the year 2000, the CAP theorem asserts that, in a distributed data store, it is impossible to simultaneously guarantee all three of the following:

1. Consistency: Every read operation in the system returns the most recent write. In other words, all nodes in the system have the same data at the same time.
2. Availability: Every request to the system receives a non-error response, without any guarantee that it contains the most recent write. The system remains responsive even in the face of failures.
3. Partition Tolerance: The system continues to operate despite the loss of messages or network partitions, ensuring that each node can operate independently.

In a distributed system, network partitions (communication breakdowns between nodes) are inevitable. The CAP theorem acknowledges that, during a network partition, a choice must be made between providing consistency or availability. This led to the famous saying that in the event of a network partition, a system can either be "consistent and partition-tolerant (CP)" or "available and partition-tolerant (AP)."

The CAP theorem is a fundamental principle in designing and understanding distributed systems, influencing architects and engineers in making decisions about system trade-offs based on the specific requirements and priorities of their applications.

CAP Theorem Media

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  Venn diagram illustrating the CAP Theorem, with intersections labeled "CA", "CP" and "AP"